Discharge lamp lighting circuit

ABSTRACT

A discharge lamp lighting circuit  1  includes an electric power supply part and a control part. The control part generates a control signal (Sc) for controlling the magnitude of electric power based on a lamp voltage (VL) of a discharge lamp. The electric power supply part supplies the electric power based on the control signal (Sc) from the control part to the discharge lamp. The control part has a differential computation part for differentiating a lamp voltage corresponding signal (VS) with respect to time and generating a first differential signal Sd 1  (=dVS/dt), and an integral computation part for integrating a second differential signal (Sd 2 ) which monotonously increases and decreases as the first differential signal (Sd 1 ) increases and decreases with respect to time and generating a first integral signal (Si 1 ), and generates the control signal (Sc) so that the electric power decreases with an increase in the first integral signal (Si 1 ).

TECHNICAL FIELD

The present disclosure relates to a discharge lamp lighting circuit.

BACKGROUND

A discharge lamp such as a metal halide lamp used in a vehicle headlightis lit in the following manner. A high-voltage pulse (e.g., several tenskV) for prompting a dielectric breakdown between electrodes is firstapplied, and a discharge arc is struck between the electrodes and theportion between the electrodes is brought into conduction. Next,relatively large electric power is supplied to increase light emissionintensity quickly. Thereafter, a voltage (lamp voltage) between theelectrodes of the discharge lamp increases as the light emissionintensity by vaporization of metal sealed inside a tube increases, sothat the supplied electric power gradually is decreased according to anincrease in this lamp voltage. In this manner, the light emissionintensity of the discharge lamp quickly converges to a predeterminedintensity while preventing an overshoot.

At present, a trace of mercury is sealed in the discharge lamp such asthe metal halide lamp. However, a (mercury-free) discharge lamp withoutcontaining mercury is being developed to prevent environmentalcontamination at the time of disposal. FIG. 14( a) is a graph showing atypical example of changes (from a start of lighting) in luminous flux(graph G10), lamp voltage (graph G11) and supply electric power (graphG12) in a conventional discharge lamp in which mercury is sealed. Also,FIG. 14( b) is a graph showing a typical example of changes (from astart of lighting) in luminous flux (graph G13), lamp voltage (graphG14) and supply electric power (graph G15) in a mercury-free dischargelamp.

In the conventional discharge lamp in which mercury is sealed, a lampvoltage immediately after starting lighting is about 27 V and graduallyincreases to about 85 V with an increase in light emission intensity asshown in FIG. 14( a). A lighting circuit decreases the supply electricpower from about 70 W to about 35 W according to a change (amount ofchange 58 V) in this lamp voltage. On the other hand, in themercury-free discharge lamp, a lamp voltage immediately after startinglighting is equal to that of the discharge lamp with mercury (about 27V) and the lamp voltage increases to about 45 V with an increase inlight emission intensity, but the amount of change (18 V) is smallerthan that of the discharge lamp with mercury as shown in FIG. 14 (b).Also, a lamp voltage value immediately after starting lighting or theamount of change in the lamp voltage with an increase in light emissionintensity has variations depending on secular change or individualdifference. When the amount of change in the lamp voltage is small, aninfluence of the variations by secular change or individual differencebecomes relatively great, so that it becomes difficult to speedilyconverge the light emission intensity while preventing an overshoot in amethod for controlling the supply electric power according to the lampvoltage value.

To address the problem of electric power supply to the mercury-freedischarge lamp as described above, a discharge lamp apparatus described,for example, in Japanese Patent Reference JP-A-2003-338390, is intendedto reduce an influence on electric power control by variations in lampvoltage of individual discharge lamps by storing a lamp voltage (lampinitial voltage) immediately after a start of lighting and controllingsupply electric power based on the amount of change in the lamp voltagefrom this lamp initial voltage.

However, the discharge lamp apparatus described in JP-A-2003-338390 canpresent the following problem. As described above, a high-voltage pulsefor prompting a dielectric breakdown between electrodes is first appliedin the case of lighting a discharge lamp. A lamp voltage immediatelyafter a start of lighting is influenced by this high-voltage pulse andbecomes unstable, so that in a method using the lamp voltage immediatelyafter the start of lighting as a lamp initial voltage, a value of thestored lamp initial voltage varies every operation and the amount ofchange in the calculated lamp voltage also varies every operation.Therefore, in the discharge lamp apparatus described inJP-A-2003-338390, it is difficult to control the supply of electricpower with good reproducibility.

SUMMARY

The invention has been implemented in view of the problem describedabove. In some implementations, the discharge lamp lighting circuitdisclosed below is capable of controlling supply electric power withgood reproducibility while suppressing an influence of variations in avoltage between electrodes by secular change or individual difference ina discharge lamp.

Among other things, in order to address the problem, a discharge lamplighting circuit is disclosed to supply electric power for lighting adischarge lamp to the discharge lamp. The circuit comprises a controlpart for generating a control signal for controlling magnitude of theelectric power based on a voltage between electrodes of the dischargelamp, and an electric power supply part for supplying the electric powerbased on the control signal from the control part to the discharge lamp.The control part has a differential computation part for differentiatinga signal according to the voltage between electrodes with respect totime and generating a first differential signal and a first integralcomputation part for integrating a second differential signal whichmonotonously increases and decreases as the first differential signalincreases and decreases with respect to time and generating a firstintegral signal, and generates the control signal so that the electricpower decreases with an increase in the first integral signal.

The present inventors found that there is a strong correlation, whichhas an extremely small influence of change with time or individualdifference in a discharge lamp, between change in light emissionintensity and a differential value and an integral value of a voltagebetween electrodes even when the amount of change in the voltage betweenelectrodes of the discharge lamp with an increase in the light emissionintensity is small and there are variations in magnitude of the voltagebetween electrodes. In the discharge lamp lighting circuit describedabove, a control part differentiates a signal according to the voltagebetween electrodes with respect to time and generates a firstdifferential signal, and integrates a second differential signal whichmonotonously increases and decreases as this first differential signalincreases and decreases with respect to time and generates a firstintegral signal, and generates a control signal so that electric powerdecreases with an increase in this first integral signal. Consequently,supply of electric power can be controlled while suppressing aninfluence of variations in the voltage between electrodes by secularchange or individual difference in the discharge lamp.

Also, in some implementations of the discharge lamp lighting circuitdescribed above, the supply electric power is controlled based on thefirst integral signal in which the second differential signal isintegrated, so that even when a voltage between electrodes immediatelyafter a start of lighting is influenced by a high-voltage pulse andvaries, an influence on electric power control can be reduced by actionof averaging the variations. Therefore, the supply electric power can becontrolled during each operation with good reproducibility.

In some implementations, the first integral computation part integratesthe first differential signal with respect to time and further generatesa second integral signal and the control part offers the control signalbased on the first integral signal to the electric power supply partafter the second integral signal reaches a first predetermined value.Consequently, the electric power control described above can be startedunder a certain condition that an integral value of the firstdifferential signal reaches the first predetermined value, so that evenwhen individual difference in a voltage between electrodes immediatelyafter a start of lighting is large, an influence of the individualdifference can be suppressed more effectively.

Also, the first integral computation part can include a first conversionpart for converting the second differential signal into a second currentsignal, a second conversion part for converting the first differentialsignal into a first current signal, a first capacitive element forcharging the first current signal and outputting a voltage across thefirst capacitive element as the second integral signal and also chargingthe second current signal and outputting a voltage across the firstcapacitive element as the first integral signal and a first currentcontrol part for controlling supply of the first and second currentsignals to the first capacitive element based on the voltage across thefirst capacitive element, and the first current control part controlsthe first and second current signals so that the first current signal isfirst supplied to the first capacitive element and the second currentsignal is supplied to the first capacitive element after the voltageacross the first capacitive element reaches the first predeterminedvalue or the corresponding value.

In some implementations, the first current signal is first supplied tothe first capacitive element and thereby, integral computation of thefirst differential signal is performed and the second integral signalcan be generated. Then, after the voltage across the first capacitiveelement (indicating the second integral signal in this case) reaches thefirst predetermined value or the corresponding value, the second currentsignal instead of the first current signal is supplied to the firstcapacitive element and thereby, integral computation of the seconddifferential signal is performed and the first integral signal can begenerated. Also, one capacitive element (first capacitive element)combines a capacitive element for integrating the first differentialsignal and generating the second integral signal with a capacitiveelement for integrating the second differential signal and generatingthe first integral signal, so that a circuit size can be reducedfurther.

Also, in some implementations, the control part has a second integralcomputation part for integrating the first differential signal withrespect to time and generating a second integral signal, and offers thecontrol signal based on the first integral signal to the electric powersupply part after the second integral signal reaches a firstpredetermined value. Consequently, the electric power control describedabove can be started under a certain condition that an integral value ofthe first differential signal reaches the first predetermined value, sothat even when individual difference in a voltage between electrodesimmediately after a start of lighting is large, an influence of theindividual difference can be suppressed more effectively.

Also, the first integral computation part can include a first conversionpart for converting the second differential signal into a second currentsignal and a first capacitive element for charging the second currentsignal and outputting a voltage across the first capacitive element asthe first integral signal, and the second integral computation partincludes a second conversion part for converting the first differentialsignal into a first current signal and a second capacitive element forcharging the first current signal and outputting a voltage across thesecond capacitive element as the second integral signal, and the controlpart further has a first current control part for controlling supply ofthe second current signal to the first capacitive element so that thesecond current signal is supplied to the first capacitive element afterthe voltage across the second capacitive element reaches the firstpredetermined value or the corresponding value.

The first differential signal can be integrated by the second capacitiveelement and the second integral signal can be generated. Then, after thevoltage across the second capacitive element (that is, the secondintegral signal) reaches the first predetermined value or thecorresponding value, the second current signal is controlled so as tosupply the second current signal to the first capacitive element andthereby, integral computation of the second differential signal isperformed and the first integral signal can be generated.

In some implementations, the first integral computation part has aresistance element connected between a constant-voltage source and thefirst capacitive element, and a second current control part forsupplying a current from the constant-voltage source to the firstcapacitive element when a voltage across the first capacitive element islarger than a second predetermined value. In this discharge lamplighting circuit, when the voltage across the first capacitive element(first integral signal) reaches the second predetermined value, acurrent from the constant-voltage source is superposed on the secondcurrent signal. That is, a signal which monotonously increases dependingon only elapsed time is superposed on the first integral signal.

When some time has elapsed since a start of lighting, a change in astate of the inside of a tube of a discharge lamp becomes small, so thatit is preferable to control the supply electric power based on theelapsed time rather than to control the supply electric power based onan integral value and a time differential value of a voltage betweenelectrodes. According to this discharge lamp lighting circuit, thesignal which monotonously increases depending on only the elapsed timeis superposed on the first integral signal and thereby, the dischargelamp can be shifted to a steady state while the supply electric power isgradually converged on target electric power and light emissionintensity close to target intensity is maintained. Further, start timingof electric power control based on the elapsed time is defined based onthe first integral signal and thereby, a gradual change in lightemission intensity in the case of shifting to the electric power controlbased on the elapsed time can be obtained.

In some implementations, the first current control part stops supply ofthe second current signal to the first capacitive element after avoltage across the first capacitive element reaches a thirdpredetermined value larger than the second predetermined value, and thethird predetermined value is less than or equal to a value of thevoltage across the first capacitive element at a point in time when thefirst differential signal becomes maximum. A discharge lamp includesmeans which exhibits characteristics in which the first differentialsignal suddenly decreases after the first differential signal becomesmaximum and means which does not exhibit the characteristics. In thisdischarge lamp lighting circuit, before the first differential signalbecomes maximum, supply of the second current signal to the firstcapacitive element is stopped and subsequently, only a current from aconstant-voltage source is integrated by the first capacitive element.Therefore, supply electric power is controlled based on only a signalwhich monotonously increases depending on only elapsed time, and aninfluence on a control signal by variations in the first differentialsignal after the first differential signal becomes maximum can beavoided.

The first integral computation part can include a function computationpart for receiving the first differential signal and generating thesecond differential signal, and the function computation part convertsthe first differential signal into the second differential signalaccording to a function having a positive first slope when magnitude ofthe first differential signal is smaller than a fourth predeterminedvalue, and converts the first differential signal into the seconddifferential signal according to a function having a positive secondslope smaller than the first slope when magnitude of the firstdifferential signal is larger than the fourth predetermined value.According to this discharge lamp lighting circuit, even in a time regionin which a voltage between electrodes suddenly increases by vaporizationof metal of the inside of a tube (that is, the first differential signalincreases), a sudden decrease in supply electric power can be preventedand a more speedup in convergence of light emission intensity can beachieved.

Various advantages can be obtained in some implementations. For example,the supply of electric power can be controlled with good reproducibilitywhile suppressing an influence of variations in a voltage betweenelectrodes by secular change or individual difference in a dischargelamp.

Other features and advantages will be apparent from the followingdetailed description, the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a first embodimentof a discharge lamp lighting circuit according to the invention.

FIG. 2 is a graph conceptually showing a relation between magnitude ofsupply electric power and a drive frequency of a transistor.

FIG. 3 is a block diagram showing a configuration of the inside andperiphery of an electric power computation part of the first embodiment.

FIG. 4 is a graph showing a relation between an output current and afirst integral signal inputted to a V/I conversion part of the firstembodiment.

FIGS. 5( a) and 5(b) are a diagram showing a relation between a lampvoltage and a current output by a current source.

FIG. 6( a) is a graph showing a situation of a change in a lamp voltagewith a lapse of time since immediately after a start of lighting, andFIG. 6( b) is a graph showing a situation of a change in a firstdifferential signal with a lapse of time since immediately after a startof lighting, and FIG. 6( c) is a graph showing a situation of a changein a voltage across a capacitive element with a lapse of time sinceimmediately after a start of lighting.

FIG. 7( a) is a graph showing a situation of a change in supply electricpower to a discharge lamp with a lapse of time since immediately after astart of lighting, and FIG. 7( b) is a graph showing a situation of achange in light emission intensity of the discharge lamp with a lapse oftime since immediately after a start of lighting.

FIG. 8 is a graph showing one example of a function of a firstdifferential signal and a second differential signal computed in afunction computation part.

FIG. 9 is a graph conceptually showing a situation of a change in a lampvoltage and a change in its time differential value in two dischargelamps with different characteristics.

FIG. 10 is a graph showing one example of time changes in light emissionintensity, a lamp voltage and a time differential value of the lampvoltage.

FIG. 11 is a circuit diagram showing a configuration example of thefunction computation part.

FIG. 12 is a block diagram showing a configuration of a control part ofa second embodiment.

FIG. 13 is a graph showing a relation between an output current and afirst integral signal inputted to a V/I conversion part of the secondembodiment.

FIG. 14( a) is a graph showing a typical example of changes (from astart of lighting) in luminous flux (graph G10), lamp voltage (graphG11) and supply electric power (graph G12) in a conventional dischargelamp in which mercury is sealed, and FIG. 14( b) is a graph showing atypical example of changes (from a start of lighting) in luminous flux(graph G13), lamp voltage (graph G14) and supply electric power (graphG15) in a mercury-free discharge lamp.

DETAILED DESCRIPTION

Preferred embodiments of a discharge lamp lighting circuit according tothe invention are described below in detail with reference to thedrawings. In addition, in the description of the drawings, the samenumerals are assigned to the same or corresponding parts.

First Embodiment

FIG. 1 is a block diagram showing an example of a configuration of afirst embodiment of a discharge lamp lighting circuit according to theinvention. A discharge lamp lighting circuit 1 shown in FIG. 1 is acircuit for supplying electric power for lighting a discharge lamp L tothe discharge lamp L, and a DC voltage from a DC power source B isconverted into an AC voltage and is supplied to the discharge lamp L.The discharge lamp lighting circuit 1 is mainly used in lamp fittingssuch as, particularly, a headlight for a vehicle. In addition, as thedischarge lamp L, for example, a mercury-free metal halide lamp issuitably used, but discharge lamps with other structures may be used.

The discharge lamp lighting circuit 1 comprises an electric power supplypart 2 for receiving power source supply from the DC power source B andsupplying AC electric power to the discharge lamp L, and a control part10 a for controlling magnitude of supply electric power to the dischargelamp L based on a voltage (hereinafter called a lamp voltage) betweenelectrodes of the discharge lamp L.

The electric power supply part 2 supplies electric power of magnitudebased on a control signal Sc from the control part 10 a described belowto the discharge lamp L. The electric power supply part 2 is connectedto the DC power source B (e.g., a battery) through a switch 20 forlighting operation, and receives a DC voltage VB from the DC powersource B and makes AC conversion and a step-up. The electric powersupply part 2 of the present embodiment has a starting circuit 3 forapplying a high-voltage pulse to the discharge lamp L at the time of astart of lighting, two transistors 5 a and 5 b, and a bridge driver 6for driving the transistors 5 a and 5 b. As the transistors 5 a and 5 b,for example, an N-channel MOSFET can be used, but other FETs or bipolartransistors may be used. In the embodiment, a drain terminal of thetransistor 5 a is connected to a plus side terminal of the DC powersource B and a source terminal of the transistor 5 a is connected to adrain terminal of the transistor 5 b and a gate terminal of thetransistor 5 a is connected to the bridge driver 6. Also, a sourceterminal of the transistor 5 b is connected to a ground potential lineGND (that is, a minus side terminal of the DC power source B) and a gateterminal of the transistor 5 b is connected to the bridge driver 6. Thebridge driver 6 alternately brings the transistors 5 a and 5 b intoconduction.

The electric power supply part 2 of the embodiment further has atransformer 7, a capacitor 8 and an inductor 9. The transformer 7 isdisposed in order to apply a high-voltage pulse to the discharge lamp Land transmit electric power and also step up the electric power. Also, aseries resonance circuit is constructed of the transformer 7, thecapacitor 8 and the inductor 9. That is, a primary winding 7 a of thetransformer 7, the inductor 9 and the capacitor 8 are mutually connectedin series. Then, one end of its series circuit is connected to thesource terminal of the transistor 5 a and the drain terminal of thetransistor 5 b and the other end is connected to the ground potentialline GND. In this configuration, a resonance frequency is determined bycapacitance of the capacitor 8 and combined reactance made of inductanceof the inductor 9 and leakage inductance of the primary winding 7 a ofthe transformer 7. In addition, the series resonance circuit isconstructed by only the primary winding 7 a and the capacitor 8 and theinductor 9 may be omitted. Also, it may be constructed so thatinductance of the primary winding 7 a is set extremely smaller than thatof the inductor 9 and a resonance frequency is substantially determinedby capacitance of the capacitor 8 and inductance of the primary winding7 a.

In the electric power supply part 2, using a series resonance phenomenonby an inductive element (an inductance component or an inductor) and thecapacitor 8, a drive frequency of the transistors 5 a and 5 b is definedat a value of this series resonance frequency or higher and thetransistors 5 a and 5 b are alternately turned on and off and ACelectric power is produced in the primary winding 7 a of the transformer7. This AC electric power is stepped up and transmitted to a secondarywinding 7 b of the transformer 7 and is supplied to the discharge lamp Lconnected to the secondary winding 7 b. In addition, the bridge driver 6for driving the transistors 5 a and 5 b drives each of the transistors 5a and 5 b reciprocally so that both the transistors 5 a and 5 b do notbecome a connection state.

Also, impedance of this series resonance circuit varies depending on thedrive frequency of the transistors 5 a and 5 b by the bridge driver 6.Therefore, magnitude of AC electric power supplied to the discharge lampL can be controlled by changing the drive frequency. Here, FIG. 2 is agraph conceptually showing a relation between magnitude of supplyelectric power and the drive frequency of the transistors 5 a and 5 b.As shown in FIG. 2, magnitude of electric power supplied to thedischarge lamp L becomes a maximum value Pmax when the drive frequencyis equal to a series resonance frequency fo, and decreases as the drivefrequency becomes higher (or becomes lower) than the series resonancefrequency fo. However, when the drive frequency is lower than the seriesresonance frequency fo, switching loss becomes large and electric powerefficiency decreases. Therefore, magnitude of a drive frequency of thebridge driver 6 is controlled in a region (region A in FIG. 2) higherthan the series resonance frequency fo. In the embodiment, the drivefrequency of the bridge driver 6 is controlled according to a pulsefrequency of the control signal Sc (signal including afrequency-modulated pulse train) from the control part 10 a connected tothe bridge driver 6.

The starting circuit 3 is a circuit for applying a high-voltage pulsefor starting to the discharge lamp L, and when trigger voltage andcurrent are applied from the starting circuit 3 to the transformer 7,the high-voltage pulse is superposed on an AC voltage generated in thesecondary winding 7 b of the transformer 7. In the starting circuit 3 ofthe embodiment, one of an output terminal is connected to the middle ofthe primary winding 7 a of the transformer 7 and the other of the outputterminal is connected to a ground potential side terminal of the primarywinding 7 a. An input voltage to the starting circuit 3 may be obtainedfrom, for example, an auxiliary winding (not shown) for starting or thesecondary winding 7 b of the transformer 7 or maybe obtained from anauxiliary winding by disposing the auxiliary winding constructing thetransformer together with the inductor 9.

The control part 10 a controls magnitude of supply electric power to thedischarge lamp L based on a lamp voltage of the discharge lamp L. Thecontrol part 10 a of the embodiment has an electric power computationpart 11 for computing magnitude of electric power to be supplied to thedischarge lamp L, an error amplifier 12 for amplifying and outputting adifference between a predetermined reference voltage and an outputvoltage Sp₁ from the electric power computation part 11, and a V-Fconversion part 13 for making voltage-frequency conversion (V-Fconversion) of a signal Sp₂ which is an analog signal output from theerror amplifier 12 and generating the control signal Sc.

The electric power computation part 11 has input ends 11 a and 11 b andan output end 11 c. The input end 11 a is connected to an intermediatetap of the secondary winding 7 b through a peak hold circuit 21 in orderto input a signal (hereinafter called a lamp voltage correspondingsignal) VS indicating magnitude of a lamp voltage VL of the dischargelamp L. The lamp voltage corresponding signal VS is set at, for example,0.35 time the peak value of the lamp voltage VL. The input end 11 b isconnected to one end of a resistance element 4 disposed for detecting alamp current of the discharge lamp L through a peak hold circuit 22 anda buffer 23. One end of the resistance element 4 is further connected toone electrode of the discharge lamp L through an output terminal of thedischarge lamp lighting circuit 1, and the other end of the resistanceelement 4 is connected to the, ground potential line GND. Then, a lampcurrent corresponding signal IS indicating magnitude of the lamp currentis output from the buffer 23. Also, the output end 11 c is connected tothe error amplifier 12.

Here, FIG. 3 is a block diagram showing a configuration of the insideand periphery of the electric power computation part 11 of theembodiment. Referring to FIG. 3, the electric power computation part 11has a differential computation part 15, an integral computation part(first integral computation part) 16, a V/I conversion part 17, andcurrent sources 18 and 19.

The differential computation part 15 is a circuit part for computing atime differential value (dVS/dt) of the lamp voltage correspondingsignal VS and generating a first differential signal Sd₁. An input end15 a of the differential computation part 15 is connected to the inputend 11 a of the electric power computation part 11. An output end 15 bof the differential computation part 15 is connected to the integralcomputation part 16. In addition, such a differential computation part15 is suitably constructed by, for example, a differentiation circuitusing the lamp voltage corresponding signal VS as input.

The integral computation part 16 is a circuit part for integrating asecond differential signal Sd₂ which monotonously increases anddecreases as the first differential signal Sd₁ increases and decreaseswith respect to time and generating a first integral signal Si₁. Aninput end 16 a of the integral computation part 16 is connected to theoutput end 15 b of the differential computation part 15. An output end16 b of the integral computation part 16 is connected to the V/Iconversion part 17.

The V/I conversion part 17 is a circuit part for subtracting a firstpredetermined value E₀ (described below) from the first integral signalSi₁ and also converting the subtracted value into a current signal I₁.An input end 17 a of the V/I conversion part 17 is connected to theoutput end 16 b of the integral computation part 16. An output end 17 bof the V/I conversion part 17 is connected to the input end 11 b of theelectric power computation part 11 through a resistance element 24. Inaddition, such a V/I conversion part 17 is suitably constructed by, forexample, a voltage-current converter and a differential amplifier usingthe first integral signal Si₁ and the predetermined value E₀ as input.

The V/I conversion part 17 outputs a current I₁ according to a functionshown in, for example, FIG. 4. That is, the V/I conversion part 17 setsthe current signal I₁ at zero when the first integral signal Si₁ is thefirst predetermined value E₀ or less, and outputs the current signal I₁of the magnitude proportional to a value obtained by subtracting E₀ fromthe first integral signal Si₁ when the first integral signal Si₁ is thefirst predetermined value E₀ or more.

The current sources 18 and 19 are a circuit part for controlling steadyelectric power (for example, 35 [W]) and supply electric power (forexample, 75 [W]) immediately after a start of lighting. Input ends 18 a,19 a of the current sources 18, 19 are connected to the input end 11 aof the electric power computation part 11. Output ends 18 b, 19 b of thecurrent sources 18, 19 are connected to one input end 12 a of the erroramplifier 12 through the output end 11 c of the electric powercomputation part 11. In addition, the other input end 12 b of the erroramplifier 12 is connected to a predetermined voltage source 14 forgenerating a predetermined reference voltage.

The current source 18 outputs a current I₂ according to a functionshown, for example, in FIG. 5( a). That is, the current source 18 setsthe current signal I₂ at zero when the lamp voltage corresponding signalVS is a certain predetermined value V₁ or less, and sets the currentsignal I₂ at a constant value when the lamp voltage corresponding signalVS is a certain predetermined value V₂ (>V₁) or more, and outputs thecurrent signal I₂ of the magnitude proportional to the lamp voltagecorresponding signal VS when the lamp voltage corresponding signal VS isV₁ or more and V₂ or less. Also, the current source 19 outputs a currentI₃ according to a function shown in, for example, FIG. 5( b). That is,the current source 19 outputs the current signal I₃ of the magnitudeproportional to the lamp voltage corresponding signal VS, and itsproportional coefficient is set so as to become small as the lampvoltage corresponding signal VS becomes high.

The integral computation part 16 is now described in further detail. Theintegral computation part 16 of the embodiment includes a functioncomputation part 161, V/I conversion parts 162 and 163, a currentcontrol part 165, and a capacitive element (first capacitive element)166.

The function computation part 161 is a circuit part for generating thesecond differential signal Sd₂ which monotonously increases anddecreases as the first differential signal Sd₁ increases and decreases.An input end 161 a of the function computation part 161 is connected tothe output end 15 b of the differential computation part 15 through theinput end 16 a of the integral computation part 16. An output end 161 bof the function computation part 161 is connected to the V/I conversionpart 162.

The V/I conversion part 162 is a first conversion part in theembodiment, and converts the second differential signal Sd₂ which is avoltage signal into a second current signal Id₂. An input end 162 a ofthe V/I conversion part 162 is connected to the output end 161 b of thefunction computation part 161. An output end 162 b of the V/I conversionpart 162 is connected to one end of the capacitive element 166 through aswitch 164 a. The other end of the capacitive element 166 is connectedto the ground potential line GND.

The V/I conversion part 163 is a second conversion part in theembodiment, and converts the first differential signal Sd₁ which is avoltage signal into a first current signal Id₁. An input end 163 a ofthe V/I conversion part 163 is connected to the output end 15 b of thedifferential computation part 15 through the input end 16 a of theintegral computation part 16. An output end 163 b of the V/I conversionpart 163 is connected to one end of the capacitive element 166 through aswitch 164 b.

The current control part 165 is a first current control part in theembodiment, and controls the first current signal Id₁ and the secondcurrent signal Id₂ based on a voltage V across the capacitive element166. The current control part 165 is constructed by including, forexample, a window comparator 165 a and a comparator 165 b. An input endof the window comparator 165 a is connected to one end of the capacitiveelement 166 and an output end is connected to a control terminal of theswitch 164 a. The window comparator 165 a outputs a voltagecorresponding to logic 0 when an input voltage (that is, the voltage Vacross the capacitive element 166) is smaller than a predetermined valueE₀ (first predetermined value) or the input voltage is larger than apredetermined value E₂ (third predetermined value), and outputs avoltage corresponding to logic 1 when the input voltage is larger thanthe predetermined value E₀ and is smaller than the predetermined valueE₂. Also, an input end of the comparator 165 b is connected to one endof the capacitive element 166 and an output end is connected to acontrol terminal of the switch 164 b. The comparator 165 b outputs avoltage corresponding to logic 1 when an input voltage (that is, thevoltage V across the capacitive element 166) is smaller than thepredetermined value E₀, and outputs a voltage corresponding to logic 0when the input voltage is larger than the predetermined value E₀. Inaddition, the switches 164 a and 164 b shall become a connection statewhen the voltage corresponding to logic 1 is inputted to the controlterminal, and become a non-connection state when the voltagecorresponding to logic 0 is inputted to the control terminal.

In addition, the current control part 165 of the embodiment controlssupply of the first current signal Id₁ and the second current signal Id₂to the capacitive element 166 by the switches 164 a and 164 b, but thecurrent control part 165 may control the second current signal Id₂ bydirectly controlling the function computation part 161 or the V/Iconversion part 162 and also may control the first current signal Id₁ bydirectly controlling the V/I conversion part 163. Also, the currentcontrol part 165 of the embodiment includes the window comparator 165 ain order to control the second current signal Id₂, but the secondcurrent signal Id₂ may be controlled using two comparators independentmutually. Also, the switches 164 a and 164 b described above aresuitably implemented by a transistor such as an FET.

The integral computation part 16 further includes a switch 167, aresistance element 168 and a comparator 169 in addition to the aboveconfiguration. The switch 167 and the resistance element 168 areconnected in series between a constant-voltage source Vcc and one end ofthe capacitive element 166. The switch 167 is suitably implemented by atransistor such as an FET. Also, the comparator 169 is a second currentcontrol part in the embodiment, and supplies a current from theconstant-voltage source Vcc to the capacitive element 166 when thevoltage V across the capacitive element 166 is larger than apredetermined value E₁ (second predetermined value). Concretely, aninput end of the comparator 169 is connected to one end of thecapacitive element 166 and an output end is connected to a controlterminal of the switch 167. The comparator 169 outputs a voltagecorresponding to logic 0 when an input voltage (that is, the voltage Vacross the capacitive element 166) is smaller than the predeterminedvalue E₁, and outputs a voltage corresponding to logic 1 when the inputvoltage is larger than the predetermined value E₁. In addition, theswitch 167 becomes a connection state when the voltage corresponding tologic 1 is inputted to the control terminal, and becomes anon-connection state when the voltage corresponding to logic 0 isinputted to the control terminal.

An operation of the discharge lamp lighting circuit 1 comprising theforegoing configuration is now described. FIGS. 6( a) to 6(c),respectively, show situations of changes in the lamp voltage VL (FIG. 6(a)), the first differential signal Sd₁ (=dVS/dt) (FIG. 6( b)) and thevoltage V across the capacitive element 166 (FIG. 6( c)) with a lapse oftime since immediately after a start of lighting. Also, FIGS. 7( a) and7(b), respectively, show situations of changes in supply electric power(FIG. 7( a)) to the discharge lamp L and light emission intensity (FIG.7( b)) of the discharge lamp L with a lapse of time since immediatelyafter a start of lighting.

First, while the bridge driver 6 shown in FIG. 1 drives the transistors5 a and 5 b at a predetermined drive frequency, a high-voltage pulse ofseveral tens kV is applied between electrodes of the discharge lamp Land prompts a dielectric breakdown by the starting circuit 3.Immediately after that, the drive frequency of the bridge driver 6 iscontrolled to a drive frequency to which the predetermined maximumelectric power (75 [W] the time of a cold start) is obtained accordingto a control signal Sc from the control part 10 a. In the control part10 a, an output voltage Sp₁, to the error amplifier 12 is controlled bycurrent signals I₂, I₃ output from the current sources 18, 19 (see FIG.3) of the electric power computation part 11. Then, V-F conversion of anoutput voltage SP₂, which is a difference between this output voltageSp₁, and a predetermined reference voltage, from the error amplifier 12is made in the V-F conversion part 13 and the output voltage SP₂ isoffered to the bridge driver 6 as the control signal Sc.

In addition, a voltage V across the capacitive element 166 of theintegral computation part 16 becomes substantially a ground potentialimmediately after a start of lighting, so that the window comparator 165a of the current control part 165 controls the switch 164 a in anon-connection state and the comparator 165 b controls the switch 164 bin a connection state. Also, the comparator 169 controls the switch 167in a non-connection state.

Subsequently, when an output signal from the differential computationpart 15 of the electric power computation part 11 becomes stable (timet₀ of FIG. 6( c)), a first differential signal Sd₁ (=dVS/dt) output fromthe differential computation part 15 is converted into a current signalId₁ in the V/I conversion part 163 of the integral computation part 16and is charged into the capacitive element 166 through the switch 164 b.Consequently, the first differential signal Sd₁ is integrated withrespect to time in the capacitive element 166. At this time, the voltageV across the capacitive element 166 is expressed by the followingmathematical formula (1) and indicates magnitude of a second integralsignal.

V=∫(dVS/dt)dt   [Mathematical formula 1]

Subsequently, when the voltage V across the capacitive element 166 (asecond integral signal in this case) reaches a predetermined value E₀(time t₁ of FIGS. 6 and 7), the switch 164 b is controlled in anon-connection state by the comparator 165 b and supply of the firstcurrent signal Id₁ to the capacitive element 166 is stopped and at thesame time, the switch 164 a is controlled in a connection state by thewindow comparator 165 a and supply of a second current signal Id₂ to thecapacitive element 166 is started. That is, the first differentialsignal Sd₁ output from the differential computation part 15 is convertedinto a second differential signal Sd₂ by the function computation part161 and the second differential signal Sd₂ is converted into the secondcurrent signal Id₂ in the V/I conversion part 162 and is charged intothe capacitive element 166 through the switch 164 a. Consequently, thesecond differential signal Sd₂ is integrated with respect to time in thecapacitive element 166. At this time, the voltage V across thecapacitive element 166 is expressed by the following mathematicalformula (2) and indicates magnitude of a first integral signal Si₁. Inaddition, in the mathematical formula (2), f(x) represents a functioncomputed in the function computation part 161.

V=∫f(dVS/dt)dt+E ₀   [Mathematical formula 2]

Here, FIG. 8 is a graph showing one example of the function f(x) of thefirst differential signal Sd₁ and the second differential signal Sd₂computed in the function computation part 161 of the embodiment. Asshown in FIG. 8, the function computation part 161 converts the firstdifferential signal Sd₁ into the second differential signal Sd₂according to a function f₁ having a positive certain slope (a firstslope which is 1 in the example of FIG. 8) when magnitude of the firstdifferential signal Sd₁ is smaller than a predetermined value (a fourthpredetermined value which is 0.3 [V/s] in the example of FIG. 8), andconverts the first differential signal Sd₁ into the second differentialsignal Sd₂ according to a function f₂ having a positive slope (a secondslope which is 0.2 in the example of FIG. 8) smaller than the firstslope when magnitude of the first differential signal Sd₁ is larger thanthe predetermined value. In addition, FIG. 8 shows a proportionalfunction as one example of the functions f₁, f₂, but the functions f₁,f₂ may be a function whose slope varies according to the firstdifferential signal Sd₁.

The voltage V across the capacitive element 166 is output from theintegral computation part 16 as the first integral signal Si₁ and isinputted to the V/I conversion part 17. Then, the predetermined value E₀(that is, the second term of the right side of the mathematical formula(2)) is subtracted from the first integral signal Si₁ and a voltagevalue after the subtraction is converted into a current signal I₁. Inthe electric power computation part 11 of the embodiment, a currentsignal I₄ formed by joining the current signal I₁ from the V/Iconversion part 17 and the current signals I₂, I₃ from the currentsources 18, 19 flows to an input end of the buffer 23 through theresistance element 24 as shown in FIG. 3. On the other hand, a lampcurrent I_(L) flows in the resistance element 4, so that a voltage dropin this resistance element 4 occurs in an output end of the buffer 23 asa lamp current corresponding signal IS. That is, the output voltage Sp₁from the electric power computation part 11 is determined by the currentsignal I₄ and the lamp current I_(L). When the first integral signal Si₁increases gradually (FIG. 6( c)), the current signal I₁ increases, sothat a voltage drop in the resistance element 24 increases and afrequency of the control signal Sc output from the V-F conversion part13 becomes high gradually. Consequently, supply electric power to thedischarge lamp L is reduced gradually (FIG. 7( a)).

Subsequently, when the voltage V across the capacitive element 166(first integral signal Si₁) reaches a predetermined value E₁ (time t₂ ofFIGS. 6 and 7), the switch 167 is controlled in a connection state bythe comparator 169. Consequently, a current from a constant-voltagesource Vcc is superposed on the second current signal Id₂ and isintegrated by the capacitive element 166 (FIG. 6( c)). That is, a signal(hereinafter called g(t)) which monotonously increases depending on onlyelapsed time is superposed on an integral value of the seconddifferential signal Sd₂ and the voltage V across the capacitive element166 becomes a value shown in the following mathematical formula (3).This voltage V across the capacitive element 166 is output as the firstintegral signal Si₁, and electric power according to this first integralsignal Si₁ is reduced from the supply electric power to the dischargelamp L (FIG. 7( a)).

V=∫f(dVS/dt)dt+g(t)+E ₀   [Mathematical formula 3]

Subsequently, when the voltage V across the capacitive element 166(first integral signal Si₁) reaches a predetermined value E₂ (>E₁) (timet₃ of FIGS. 6 and 7), the switch 164 a is controlled in a non-connectionstate by the window comparator 165 a and supply of the second currentsignal Id₂ to the capacitive element 166 is stopped. Consequently, onlythe current from the constant-voltage source Vcc is supplied to thecapacitive element 166. That is, the supply electric power to thedischarge lamp L is reduced according to only a time function g(t) andgradually converges on target electric power (for example, 35 [W]) (FIG.7( a)). In addition, it is preferable that the predetermined value E₂ beless than or equal to the voltage V across the capacitive element 166 (avalue E₃ shown in FIG. 6( c)) at a point in time when the firstdifferential signal Sd₁ becomes maximum.

Effects obtained by the discharge lamp lighting circuit 1 of theembodiment described above are as follows. As described in theBackground section, in a mercury-free discharge lamp, the amount ofchange in a lamp voltage since immediately after a start of lighting isas small as about 18 [V] and an influence of variations by secularchange or individual difference becomes relatively large. The presentinventors found that there is a strong correlation, which has anextremely small influence of change with time or individual difference,between change in light emission intensity and a differential value andan integral value of a lamp voltage even when the amount of change inthe lamp voltage is small and there are variations in magnitude of thelamp voltage. In the discharge lamp lighting circuit 1 of theembodiment, the control part 10 a differentiates the lamp voltagecorresponding signal VS with respect to time and generates the firstdifferential signal Sd₁, and integrates the second differential signalSd₂ which monotonously increases and decreases as this firstdifferential signal Sd₁ increases and decreases with respect to time andgenerates the first integral signal Si₁, and generates the controlsignal Sc so that a drive frequency becomes high (that is, supplyelectric power decreases) with an increase in this first integral signalSi₁. Consequently, the supply electric power can be controlled suitablywhile suppressing an influence of variations in the lamp voltage VL bysecular change or individual difference in the discharge lamp L.

Also, the control part 10 a of the embodiment controls the supplyelectric power based on the first integral signal Si₁ in which thesecond differential signal Sd₂ is integrated, so that even when the lampvoltage VL immediately after a start of lighting is influenced by ahigh-voltage pulse from the starting circuit 3 and varies, an influenceon electric power control can be reduced by action of averaging thevariations. Therefore, according to the discharge lamp lighting circuit1 of the embodiment, the supply electric power can be controlled everyoperation with good reproducibility.

Also, as shown in the embodiment, the integral computation part 16preferably integrates the first differential signal Sd₁ with respect totime and generates a second integral signal and the control part 10 aoffers the control signal Sc based on the first integral signal Si₁ tothe electric power supply part 2 after the second integral signalreaches the predetermined value E₀. Consequently, electric power controlbased on the first integral signal Si₁ can be started under a certaincondition that an integral value (second integral signal) of the firstdifferential signal Sd₁ reaches the predetermined value E₀, so that evenwhen individual difference in the lamp voltage VL immediately after astart of lighting is large, an influence of the individual differencecan be suppressed more effectively.

Also, as shown in the embodiment, the integral computation part 16 ispreferably constructed by including the V/I conversion parts 162 and163, the current control part 165, and the capacitive element 166. Then,the current control part 165 preferably controls the first and secondcurrent signals Id₁ and Id₂ so that the first current signal Id₁ isfirst supplied to the capacitive element 166 and the second currentsignal Id₂ is supplied to the capacitive element 166 after the voltage Vacross the capacitive element 166 reaches the predetermined value E₀.

Thus, the first current signal Id₁ is first supplied to the capacitiveelement 166 and thereby, integral computation of the first differentialsignal Sd₁ is performed and the second integral signal can be generatedsuitably. Then, after the voltage V across the capacitive element 166(second integral signal) reaches the predetermined value E₀, the secondcurrent signal Id₂ instead of the first current signal Id₁ is suppliedto the capacitive element 166 and thereby, integral computation of thesecond differential signal Sd₂ is performed and the first integralsignal Si₁ can be generated suitably. According to the integralcomputation part 16 thus, one capacitive element 166 combines acapacitive element for integrating the first differential signal Sd₁ andgenerating the second integral signal with a capacitive element forintegrating the second differential signal Sd₂ and generating the firstintegral signal Si₁, so that a circuit size can be reduced further.

Also, as shown in the embodiment, the integral computation part 16preferably has the resistance element 168 connected between theconstant-voltage source Vcc and the capacitive element 166, and thesecond current control part (comparator 169) for supplying a currentfrom the constant-voltage source Vcc to the capacitive element 166 whenthe voltage V across the capacitive element 166 (first integral signalSi₁) is larger than the predetermined value E₁. Then, when the voltage Vacross the capacitive element 166 (first integral signal Si₁) reachesthe predetermined value E₁, the signal g(t) which monotonously increasesdepending on only elapsed time is preferably superposed on the firstintegral signal Si₁.

At an initial stage of a start of lighting, a change in a state of theinside of a tube of the discharge lamp L is great, so that supplyelectric power is controlled based on an integral value and a timedifferential value of the lamp voltage VL (an integral value and a timedifferential value (dVS/dt) of the lamp voltage corresponding signal VSin the embodiment) with a high correlation to light emission intensityand thereby, variations in the lamp voltage VL are accommodated and thesupply electric power can be controlled suitably. However, when sometime has elapsed since a start of lighting, the change in the state ofthe inside of the tube of the discharge lamp L becomes small, so that itis preferable to control the supply electric power based on elapsed timerather than to control the supply electric power based on the integralvalue and the time differential value of the lamp voltage VL. Accordingto the discharge lamp lighting circuit 1 of the embodiment, the signalg(t) which monotonously increases depending on only the elapsed time issuperposed on the first integral signal Si₁ and thereby, the dischargelamp L can be shifted to a steady state while the supply electric poweris gradually converged on target electric power and light emissionintensity close to target intensity is maintained. Further, start timingof electric power control based on the elapsed time is defined(predetermined value E₁) based on the first integral signal Si₁ andthereby, a gradual change in light emission intensity in the case ofshifting to the electric power control based on the elapsed time can beobtained.

Also, when the integral computation part 16 has the resistance element168 and the comparator 169, the current control part 165 preferablystops supply of the second current signal Id₂ to the capacitive element166 after the voltage V across the capacitive element 166 reaches thepredetermined value E₂ larger than the predetermined value E₁ as shownin the embodiment. Then, the predetermined value E₂ is preferably lessthan or equal to the voltage V across the capacitive element 166 (firstintegral signal Si₁) at a point in time when the first differentialsignal Sd₁ becomes maximum.

Here, FIG. 9 is a graph conceptually showing a situation of a change inthe lamp voltage VL and a change in its time differential value (dVL/dt)in two discharge lamps with different characteristics. In addition, inFIG. 9, the axis of ordinate shows the lamp voltage VL or its timedifferential value and the axis of abscissa shows elapsed time since astart of lighting. Also, graphs G1 and G2 respectively show a lampvoltage VL of a certain discharge lamp and its time differential value,and graphs G3 and G4 respectively show a lamp voltage VL of anotherdischarge lamp and its time differential value. As shown in the graphs,the discharge lamp includes means (graph G4) which exhibitscharacteristics in which a time differential value of the lamp voltageVL suddenly decreases after the time differential value becomes maximum,and means (graph G2) which exhibits characteristics in which the timedifferential value decreases relatively gradually. If electric powercontrol based on the second differential signal Sd₂ is continued, lightemission intensity of the discharge lamp having the characteristics asshown in graphs G3 and G4 may overshoot when the control part 10 a isadjusted using the discharge lamp having the characteristics as shown ingraphs G1 and G2. In reverse, when the control part 10 a is adjustedusing the discharge lamp having the characteristics as shown in graphsG3 and G4, light emission intensity of the discharge lamp having thecharacteristics as shown in graphs G1 and G2 may undershoot.

On the other hand, in the discharge lamp lighting circuit 1 of theembodiment, before the first differential signal Sd₁ becomes maximum,supply of the second current signal Id₂ to the capacitive element 166 isstopped and subsequently, only a current from the constant-voltagesource Vcc is integrated by the capacitive element 166. Therefore,supply electric power is controlled based on only the signal g(t) whichmonotonously increases depending on only elapsed time, and an influenceon the control signal Sc by variations in the first differential signalSd₁ after the first differential signal Sd₁ becomes maximum can beavoided.

Also, as shown in FIG. 8, the function computation part 161 of theintegral computation part 16 preferably converts the first differentialsignal Sd₁ into the second differential signal Sd₂ according to thefunction f₁ having a positive certain slope when magnitude of the firstdifferential signal Sd₁ is smaller than a certain predetermined value,and converts the first differential signal Sd₁ into the seconddifferential signal Sd₂ according to the function f₂ having a positivesmaller slope when magnitude of the first differential signal Sd₁ islarger than the predetermined value. If supply electric power iscontrolled based on the first differential signal Sd₁ without makingconversion by the function computation part 161, as shown in FIG. 10, ina time zone in which light emission intensity suddenly increases byvaporization of metal of the inside of a tube, that is, in the time zone(a region C in FIG. 10) in which the lamp voltage VL suddenly increases,the supply electric power is reduced more than necessary, with theresult that a rise in light emission intensity is delayed. On the otherhand, when the first differential signal Sd₁ (=dVS/dt) exceeds a certainpredetermined value, the functions f₁, f₂ in which an increase in acurrent signal to the capacitive element 166 is suppressed are appliedto the first differential signal Sd₁ and thereby, even in a time regionin which the lamp voltage VL suddenly increases (that is, the firstdifferential signal Sd₁ increases), the supply electric power can beprevented from being reduced more than necessary and a more speedup inconvergence of light emission intensity can be achieved.

A concrete example of the function computation part 161 according to thefirst embodiment is now described. In addition, the following example isone example of a concrete circuit configuration for implementing thefunction computation part 161 according to the embodiment, and thefunction computation part 161 can also be implemented by circuitconfigurations other than the following circuit configuration.

FIG. 11 is a circuit diagram showing a configuration example of thefunction computation part 161. Referring to FIG. 11, this functioncomputation part 161 has an amplification circuit 201, output controlcircuits 202 and 203, and a suction buffer circuit 204. Theamplification circuit 201 includes an amplifier 211. A non-invertinginput end 211 a of the amplifier 211 is connected to an input end 161 aof the function computation part 161. An inverting input end 211 b ofthe amplifier 211 is connected to an output end 211 c of the amplifier211 through a resistance element 212, and is grounded through aresistance element 213. Also, the inverting input end 211 b is connectedto an output end 201 a of the amplification circuit 201.

The output control circuit 202 has a NOR circuit 216 and a transistor214 such as an FET. A drain terminal of the transistor 214 is connectedto the output end 201 a of the amplification circuit 201. A sourceterminal of the transistor 214 is grounded and a gate terminal isconnected to an output end of the NOR circuit 216 through a resistanceelement 215. One input end of the NOR circuit 216 is connected to anoutput end of a comparator 165 c. In addition, the comparator 165 c isone comparator in the case of dividing the window comparator 165 a ofthe first embodiment into two independent comparators, and outputs avoltage corresponding to logic 1 when a voltage V across the capacitiveelement 166 (see FIG. 3) is larger than a predetermined value E₀. Asignal S_(VL) which becomes logic 1 when a lamp voltage VL exceeds acertain reference value is inputted to the other input end of the NORcircuit 216.

The output control circuit 203 has a transistor 221 such as an FET. Adrain terminal of the transistor 221 is connected to the output end 201a of the amplification circuit 201. A source terminal of the transistor221 is grounded and a gate terminal is connected to an output end of acomparator 165 d through a resistance element 222. In addition, thecomparator 165 d is the other comparator in the case of dividing thewindow comparator 165 a of the first embodiment into two independentcomparators, and outputs a voltage corresponding to logic 1 when avoltage V across the capacitive element 166 (see FIG. 3) is larger thana predetermined value E₂.

The suction buffer circuit 204 has an amplifier 231 and a diode 232. Apredetermined voltage E₄ (corresponding to a fourth predetermined value)in which resistance voltage division is made is inputted to anon-inverting input end 231 a of the amplifier 231. An inverting inputend 231 b of the amplifier 231 is connected to an anode of the diode232, and an output end 231 c of the amplifier 231 is connected to acathode of the diode 232. Also, the anode of the diode 232 is connectedto the output end 201 a of the amplification circuit 201 through aresistance element 233 and a resistance element 218. In addition, apoint of connection between the resistance element 233 and theresistance element 218 is connected to an output end 161 b of thefunction computation part 161.

When the voltage V across the capacitive element 166 exceeds thepredetermined value E₀ (corresponding to time t₁ of FIG. 6( c)) in thisfunction computation part 161, the transistor 214 becomes anon-connection state and a potential according to a first differentialsignal Sd₁ develops in the output end 201 a of the amplification circuit201. At this time, while the potential of the output end 201 a of theamplification circuit 201 is the predetermined voltage E₄ or less, theamplifier 231 attempts to pass a current through the resistance elements233 and 218, but the current is blocked by the diode 232. Therefore, apotential (second differential signal Sd₂) of the output end 161 bbecomes almost equal to the first differential signal Sd₁ (correspondingto the function f₁ shown in FIG. 8). Thereafter, when the potential ofthe output end 201 a of the amplification circuit 201 exceeds thepredetermined voltage E₄, the suction buffer circuit 204 sucks a currentthrough the resistance elements 218 and 233, so that a value of thesecond differential signal Sd₂ becomes a value shown in the followingmathematical formula (4) (corresponding to the function f₂ shown in FIG.8).

$\begin{matrix}\begin{matrix}{{Sd}_{2} = {E_{4} + {\left( {{Sd}_{1} - E_{4}} \right) \cdot {R_{233}/}}}} \\{\left( {R_{233} + R_{218}} \right)} \\{= {{{Sd}_{1} \cdot {R_{233}/\left( {R_{233} + R_{218}} \right)}} +}} \\{{E_{4} \cdot {R_{218}/\left( {R_{233} + R_{218}} \right)}}}\end{matrix} & \left\lbrack {{Mathematical}\mspace{14mu} {formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In addition, in the mathematical formula (4), R₂₁₈ and R₂₃₃ respectivelyrepresent resistance values of the resistance elements 218 and 233.Thereafter, when the voltage V across the capacitive element 166 exceedsthe predetermined value E₂, the transistor 221 becomes a connectionstate and the output end 201 a of the amplification circuit 201 isgrounded and a signal output from the output end 161 b is stopped.

Second Embodiment

Next, another example of a control part will be described as a secondembodiment of a discharge lamp lighting circuit according to theinvention. FIG. 12 is a block diagram showing a configuration of acontrol part 10 b of the present embodiment. The control part 10 b ofthe embodiment has an electric power computation part 31 instead of theelectric power computation part 11 of the first embodiment. The electricpower computation part 31 has input ends 31 a and 31 b and an output end31 c. The input end 31 a is connected to an intermediate tap of asecondary winding 7 b (see FIG. 1) through a peak hold circuit 21. Theinput end 31 b is connected to one end of a resistance element 4 (seeFIG. 1) disposed for detecting a lamp current IS of a discharge lamp Lthrough a peak hold circuit 22 and a buffer 23. The output end 31 c isconnected to an input end 12 a of an error amplifier 12.

The electric power computation part 31 has a differential computationpart 15, a first integral computation part 32, a second integralcomputation part 33, a current control part 34, a V/I conversion part35, and current sources 18 and 19. The differential computation part 15and the current sources 18 and 19 among them are similar to those of thefirst embodiment, so that detailed description is omitted.

The first integral computation part 32 is a circuit part for integratinga second differential signal Sd₂ based on a first differential signalSd₁ inputted from the differential computation part 15 with respect totime and generating a first integral signal Si₁. An input end 32 a ofthe integral computation part 32 is connected to an output end 15 b ofthe differential computation part 15. An output end 32 b of the integralcomputation part 32 is connected to the V/I conversion part 35.

The first integral computation part 32 includes a function computationpart 161, a V/I conversion part 162 (first conversion part), a switch164 a, a capacitive element (first capacitive element) 166, a switch167, a resistance element 168, and a comparator 169 (second currentcontrol part). These configurations are similar to those of the firstembodiment.

The second integral computation part 33 is a circuit part forintegrating a first differential signal Sd₁ with respect to time andgenerating a second integral signal Si₂. The second integral computationpart 33 includes a V/I conversion part 331 (second conversion part) forconverting the first differential signal Sd₁ which is a voltage signalinto a first current signal Id₁, and a capacitive element 332 (secondcapacitive element) for charging the first current signal Id₁. An inputend 331 a of the V/I conversion part 331 is connected to the output end15 b of the differential computation part 15. An output end 331 b of theV/I conversion part 331 is connected to one end of the capacitiveelement 332. In addition, the other end of the capacitive element 332 isgrounded.

The current control part 34 is a first current control part in theembodiment, and controls supply of a second current signal Id₂ to thecapacitive element 166 based on a voltage V across the capacitiveelement 166 (first integral signal Si₁) and a voltage across thecapacitive element 332 (second integral signal Si₂). The current controlpart 34 is constructed by including, for example, comparators 341 and342 and an AND circuit 343. An input end of the comparator 341 isconnected to one end of the capacitive element 332 of the secondintegral computation part 33 and an output end is connected to one inputend of the AND circuit 343. The comparator 341 outputs a voltagecorresponding to logic 0 when an input voltage (that is, the voltageacross the capacitive element 332) is smaller than a predetermined valueE₀ (first predetermined value), and outputs a voltage corresponding tologic 1 when the input voltage is larger than the predetermined valueE₀. Also, an input end of the comparator 342 is connected to one end ofthe capacitive element 166 and an output end is connected to the otherinput end of the AND circuit 343. The comparator 342 outputs a voltagecorresponding to logic 1 when an input voltage (that is, the voltage Vacross the capacitive element 166) is smaller than a predetermined valueE₂, and outputs a voltage corresponding to logic 0 when the inputvoltage is larger than the predetermined value E₀. In addition, anoutput end of the AND circuit 343 is connected to a control terminal ofthe switch 164 a. The switch 164 a becomes a connection state when thevoltage corresponding to logic 1 is inputted to the control terminal,and becomes a non-connection state when the voltage corresponding tologic 0 is inputted to the control terminal.

In addition, the current control part 34 of the embodiment controlssupply of the second current signal Id₂ to the capacitive element 166 bythe switch 164 a, but the current control part 34 may control the secondcurrent signal Id₂ by directly controlling the function computation part161 or the V/I conversion part 162.

The V/I conversion part 35 is a circuit part for converting the firstintegral signal Si₁ into a current signal I₁. An input end 35 a of theV/I conversion part 35 is connected to the output end 32 b of the firstintegral computation part 32. An output end 35 b of the V/I conversionpart 35 is connected to the input end 31 b of the electric powercomputation part 31 through a resistance element 24. The V/I conversionpart 35 outputs the current I₁ according to, for example, a functionshown in FIG. 13. That is, the V/I conversion part 35 outputs thecurrent signal I₁ of magnitude proportional to the first integral signalSi₁.

An operation of the electric power computation part 31 comprising theabove configuration will be described again with reference to FIGS. 6and 7. When an output signal from the differential computation part 15becomes stable after a start of lighting (time to of FIG. 6( c)), afirst differential signal Sd₁ (=dVS/dt) output from the differentialcomputation part 15 is converted into a current signal Id₁ in the V/Iconversion part 331 of the second integral computation part 33 and ischarged into the capacitive element 332. Consequently, the firstdifferential signal Sd₁ is integrated with respect to time in thecapacitive element 332 and a second integral signal Si₂ is generated.

Subsequently, when a voltage across the capacitive element 332 (that is,the second integral signal Si₂) reaches a predetermined value E₀ (timet₁ of FIGS. 6 and 7), an output of the comparator 341 becomes logic 1and the switch 164 a is controlled in a connection state and supply of asecond current signal Id₂ to the capacitive element 166 is started. Thatis, the first differential signal Sd₁ output from the differentialcomputation part 15 is converted into a second differential signal Sd₂by the function computation part 161 and the second differential signalSd₂ is converted into a second current signal Id₂ in the V/I conversionpart 162 and is charged into the capacitive element 166 through theswitch 164 a. Consequently, the second differential signal Sd₂ isintegrated with respect to time in the capacitive element 166 and afirst integral signal Si₁ is generated.

The first integral signal Si₁ is output from the first integralcomputation part 32 and is inputted to the V/I conversion part 35. Then,the first integral signal Si₁ is converted into the current signal I₁ inthe V/I conversion part 35. When the first integral signal Si₁ increasesgradually (FIG. 6( c)), the current signal I₁ increases, so that avoltage drop in the resistance element 24 increases and a frequency of acontrol signal Sc output from the V-F conversion part 13 (see FIG. 1)becomes high gradually. Consequently, supply electric power to thedischarge lamp L is reduced gradually (FIG. 7( a)).

Subsequently, when a voltage V across the capacitive element 166 (firstintegral signal Si₁) reaches a predetermined value E₁ (time t₂ of FIGS.6 and 7), the switch 167 is controlled in a connection state by thecomparator 169. Consequently, a current from a constant-voltage sourceVcc is superposed on the second current signal Id₂ and the voltage Vacross the capacitive element 166 becomes a value in which a timefunction g(t) is superposed on an integral value of the seconddifferential signal Sd₂. This voltage V across the capacitive element166 is output as the first integral signal Si₁, and electric poweraccording to this first integral signal Si₁ is reduced from the supplyelectric power to the discharge lamp L (FIG. 7( a)).

Subsequently, when the voltage V across the capacitive element 166(first integral signal Si₁) reaches a predetermined value E₂ (>E₁) (timet₃ of FIGS. 6 and 7), an output of the comparator 342 becomes logic 0and the switch 164 a is controlled in a non-connection state and supplyof the second current signal Id₂ to the capacitive element 166 isstopped. Consequently, only the current from the constant-voltage sourceVcc is supplied to the capacitive element 166, and the supply electricpower to the discharge lamp L is reduced according to only the timefunction g(t) and gradually converges on target electric power (forexample, 35 [W]) (FIG. 7( a)).

Effects that can be obtained by some implementations of the dischargelamp lighting circuit (control part 10 b) of the embodiment describedabove are as follows. The supply electric power can be controlled whilesuppressing an influence of variations in the lamp voltage VL by secularchange or individual difference in the discharge lamp L in a mannersimilar to the first embodiment. Also, even when the lamp voltage VLimmediately after a start of lighting is influenced by a high-voltagepulse from the starting circuit 3 and varies, an influence on electricpower control can be reduced by action of averaging the variations andthe supply electric power can be controlled every operation with goodreproducibility.

Also, as shown in the embodiment, the control part 10 b may have thesecond integral computation part 33 for integrating the firstdifferential signal Sd₁ with respect to time and generating the secondintegral signal Si₂, and may offer the control signal Sc based on thefirst integral signal Si₁ to the electric power supply part 2 (seeFIG. 1) after the second integral signal Si₂ reaches the predeterminedvalue E₀. Consequently, electric power control based on the firstintegral signal Si₁ can be started under a certain condition that anintegral value (second integral signal Si₂) of the first differentialsignal Sd₁ reaches the predetermined value E₀, so that even whenindividual difference in the lamp voltage VL immediately after a startof lighting is large, an influence of the individual difference can besuppressed more effectively.

Also, as shown in the embodiment, the first integral computation part 32may include the V/I conversion part 162 and the capacitive element 166,and the second integral computation part 33 may include the V/Iconversion part 331 and the capacitive element 332, and the currentcontrol part 34 may control the second current signal Id₂ so that thesecond current signal Id₂ is supplied to the capacitive element 166after the voltage across the capacitive element 332 (that is, the secondintegral signal Si₂) reaches the predetermined value E₀.

By this configuration, the first differential signal Sd₁ is integratedby the capacitive element 332 and the second integral signal Si₂ can begenerated. Then, the second current signal Id₂ is controlled so that thesecond current signal Id₂ is supplied to the capacitive element 166after the voltage across the capacitive element 332 (that is, the secondintegral signal Si₂) reaches the predetermined value E₀ and thereby,integral computation of the second differential signal Sd₂ is performedand the first integral signal Si₁ can be generated.

The discharge lamp lighting circuit according to the invention is notlimited to the specific embodiments described above, and variousmodifications can be made. For example, in the each of the embodimentsdescribed above, the control part (particularly, the electric powercomputation part) has been constructed by an analog circuit, but thecontrol part (particularly, the electric power computation part)according to the invention may be implemented by executing predeterminedsoftware in a computer having a CPU and memory.

Other implementations are within the scope of the claims.

1. A discharge lamp lighting circuit for supplying electric power tolight a discharge lamp, the discharge lamp lighting circuit comprising:control circuitry to generate a control signal for controlling amagnitude of the electric power based on a voltage between electrodes ofthe discharge lamp, and electric power supply circuitry to supply theelectric power to the discharge lamp based on the control signal fromthe control circuitry, wherein the control circuitry comprises:differential computation circuitry to differentiate a signal accordingto the voltage between the electrodes with respect to time, and togenerate a first differential signal, and first integral computationcircuitry to integrate a second differential signal which monotonouslyincreases and decreases as the first differential signal increases anddecreases with respect to time, and to generate a first integral signal,wherein the control circuitry is operable to generate the control signalso that the electric power decreases with an increase in the firstintegral signal.
 2. A discharge lamp lighting circuit as claimed inclaim 1, wherein the first integral computation circuitry is operable tointegrate the first differential signal with respect to time and togenerate a second integral signal, and the control circuitry is operableto provide the control signal to the electric power supply circuitrybased on the first integral signal after the second integral signalreaches a first predetermined value.
 3. A discharge lamp lightingcircuit as claimed in claim 2, wherein the first integral computationcircuitry includes first conversion circuitry to convert the seconddifferential signal into a second current signal, second conversioncircuitry to convert the first differential signal into a first currentsignal, a first capacitive element to charge the first current signaland output a voltage across the first capacitive element as the secondintegral signal and to charge the second current signal and output avoltage across the first capacitive element as the first integralsignal, and first current control circuitry to control supply of thefirst and second current signals to the first capacitive element basedon the voltage across the first capacitive element, wherein the firstcurrent control circuitry is operable to control the first and secondcurrent signals so that the first current signal is first supplied tothe first capacitive element and the second current signal is suppliedto the first capacitive element after the voltage across the firstcapacitive element reaches the first predetermined value or thecorresponding value.
 4. A discharge lamp lighting circuit as claimed inclaim 1, wherein the control circuitry further has second integralcomputation circuitry to integrate the first differential signal withrespect to time and to generate a second integral signal, and to providethe control signal to the electric power supply circuitry based on thefirst integral signal after the second integral signal reaches a firstpredetermined value.
 5. A discharge lamp lighting circuit as claimed inclaim 4, wherein the first integral computation circuitry includes firstconversion circuitry to convert the second differential signal into asecond current signal and a first capacitive element to charge thesecond current signal and output a voltage across the first capacitiveelement as the first integral signal, and wherein the second integralcomputation circuitry includes second conversion circuitry to convertthe first differential signal into a first current signal and a secondcapacitive element to charge the first current signal and output avoltage across the second capacitive element as the second integralsignal, and wherein the control circuitry further has first currentcontrol circuitry to control supply of the second current signal to thefirst capacitive element so that the second current signal is suppliedto the first capacitive element after the voltage across the secondcapacitive element reaches the first predetermined value or thecorresponding value.
 6. A discharge lamp lighting circuit as claimed inclaim 3, wherein the first integral computation circuitry includes: aresistance element connected between a constant-voltage source and thefirst capacitive element, and second current control circuitry to supplya current from the constant-voltage source to the first capacitiveelement when a voltage across the first capacitive element is largerthan a second predetermined value.
 7. A discharge lamp lighting circuitas claimed in claim 6, wherein the first current control circuitry isoperable to stop supply of the second current signal to the firstcapacitive element after a voltage across the first capacitive elementreaches a third predetermined value larger than the second predeterminedvalue, and the third predetermined value is less than or equal to avalue of the voltage across the first capacitive element when the firstdifferential signal reaches its maximum value.
 8. A discharge lamplighting circuit as claimed in claim 5, wherein the first integralcomputation circuitry includes: a resistance element connected between aconstant-voltage source and the first capacitive element, and secondcurrent control circuitry to supply a current from the constant-voltagesource to the first capacitive element when a voltage across the firstcapacitive element is larger than a second predetermined value.
 9. Adischarge lamp lighting circuit as claimed in claim 8, wherein the firstcurrent control circuitry is operable to stop supply of the secondcurrent signal to the first capacitive element after a voltage acrossthe first capacitive element reaches a third predetermined value largerthan the second predetermined value, and the third predetermined valueis less than or equal to a value of the voltage across the firstcapacitive element when the first differential signal reaches itsmaximum value.
 10. A discharge lamp lighting circuit as claimed in claim1, wherein the first integral computation circuitry includes functioncomputation circuitry to receive the first differential signal and togenerate the second differential signal, and wherein the functioncomputation circuitry is operable to convert the first differentialsignal into the second differential signal according to a functionhaving a positive first slope when a magnitude of the first differentialsignal is smaller than a fourth predetermined value, and to convert thefirst differential signal into the second differential signal accordingto a function having a positive second slope smaller than the firstslope when the magnitude of the first differential signal is larger thanthe fourth predetermined value.